Method and apparatus for a digitized CATV network for bundled services

ABSTRACT

A cost-efficient digital CATV network to improve signal quality, provide reliability, and offer the ability to meet demands for interactive services is described. Analog or digital video downstream channels are converted to a digital format by a digital headend transmitter. Relatively costly error-encoding for digital video channels is also part of the digital headend transmitter. Downstream analog and digital video channels in the digital format are transmitted using time-division multiplex technology from a headend to nodes using standard network protocols, such as SONET. Standard network protocols provide error-monitoring and status indication of transmit data, thus ensuring high signal quality and reliability. Time-division multiplexing facilitates easy adding or dropping of information to a transmit path. Flexibility to add or drop information is critical in providing interactive services. Data from interactive services can be added or dropped at points of presence throughout the digital CATV network. Subscribers to the digital CATV network can communicate with each other. A digital node transmitter receives the analog or digital video channels in digital format and converts the analog or digital video channels into an analog format. The digital node transmitter also frequency-division multiplexes multiple analog or digital video channels into one analog broadband signal for broadcast to subscribers&#39; homes.

RELATED APPLICATION

The present application claims priority to co-pending provisionalapplication entitled METHOD AND APPARATUS FOR A DIGITIZED CATV NETWORKFOR BUNDLED SERVICES, application Serial No. 60-181-133, filed Feb. 8,2000, which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to digital signal processing anddigital networks, and more specifically to distribution of signals overa digital cable television network.

2. Description of the Related Art

There is a growing demand for a cable television (CATV) network tosupport a wide variety of services: analog video, digital video,interactive video, high-speed data access, telephony, and telemetry.Bundled services, supplying multiple services simultaneously, aredesired. In order to meet the demand, the CATV network must be able tooffer high signal quality over long distances, offer flexibility inadding or dropping services, provide network reliability, and providecost efficiency.

Presently, information gathering equipment resides in a headend.Equipment used to process the gathered information and configure theinformation for reception by subscribers also resides in the headend. Ina typical CATV network, information from various sources, includingsatellite or video feed is received at the headend for broadcast in theCATV network. The information received may be legacy analog videochannels operating at an Intermediate Frequency (IF) or digitallyencoded video channels (e.g., Moving Picture Experts Group (MPEG) data).CATV broadcast signals are transmitted from the headend to subscribersin an analog format over a designated frequency bandwidth. A transmitterat the headend frequency-division multiplexes the video channels beforebroadcasting to multiple nodes. Each analog video channel is modulatedonto its designated radio frequency carrier. The digital bitstream ofeach digital video channel is error-encoded, modulated, and converted toan analog signal before modulation onto its designated radio frequencycarrier.

The analog nature of the broadcast signal limits the transmissiondistance from the headend to the nodes being served. The CATV network istypically a Hybrid-Fiber-Coax (HFC) system. The broadcast signal isoften transmitted from the headend to the nodes using fiber opticcables. The broadcast signal is transmitted from the node to subscribersusing coaxial cables. The quality of the analog signal can besufficiently maintained in the range of 65 kilometers of fiber opticcable. Inherent non-linear characteristics, transmission of multiplechannels simultaneously, and noise generated throughout the CATV networksignificantly degrade the analog signal beyond the 65 kilometers rangelimit.

An alternate architecture for the CATV network is a Multiplexed FiberPassive Coax (MFPC) system. In the MFPC system, the broadcast signal isfirst transmitted from the headend to mux fiber nodes. The broadcastsignal is then transmitted from the mux fiber nodes to mini fiber nodes.Both transmissions use fiber optic cables. The broadcast signal istransmitted from the mini fiber nodes to subscribers using coaxialcables. The mini fiber nodes function similarly to the nodes in the HFCsystem. However, each node typically services a heavier load (e.g., 500to 2000 subscribers) in comparison to each mini fiber node (e.g., 50 to80 subscribers). The MFPC system is an improvement over the HFC system.The MFPC system uses shorter coaxial cables to transmit signals from thefiber system to the subscriber. Shorter coaxial cables result inincreased bandwidth capacity. Amplifiers in the coaxial cabletransmission path are eliminated. Power can be delivered to subscriberequipment via the coaxial cables.

The present CATV network, using either the HFC or the MFPC system, is anopen-loop system. The broadcast signals in an analog format are sentfrom the headend to the nodes, which in turn send the signals to thesubscribers. The quality of the signal is not known until it reaches thesubscriber. Errors caused by distortion, noise, or faulty equipment arenot automatically monitored. The current CATV network is 95% reliable.However, interactive services require 99.9% reliability.

SUMMARY OF THE INVENTION

The present invention solves these and other problems by providing acost-effective and flexible digital CATV network wherein a headendtransmitter receives signals and produces a digital signal in a digitalformat and a node transmitter receives the digital signal in the digitalformat and produces an output in an analog format. In the existing CATVnetworks, signals are transmitted in the analog format.

In the digital CATV network, video signals are in a digital format fortransmission from a headend to nodes in a cable distribution system. Thenodes convert the digital data to an analog format for distribution tosubscribers. Subscribers include homes, schools, businesses, andgovernment agencies. In this application, the term home is synonymouswith the term subscriber. The digital CATV network drastically improvessignal quality as transmission of digital signals do not require ahighly linear network. Digital signals can tolerate higher noise levelsthan analog signals. The quality of digital signals can be sufficientlymaintained in transmission through thousands of kilometers of fiberoptic cable by spacing repeaters or optical amplifiers in thetransmission path (e.g., every 100 kilometers) to relay the digitalsignals.

In one embodiment, a digital transmitter at a headend digitizes eachanalog video channel and frames the digital data into a SynchronousOptical NETwork (SONET) Optical Carrier level 3c (OC-3c) bitstream. Theelectrical equivalent of OC-N is Synchronous Transport Signal level N(STS-N). In this application, the terms OC and STS are usedinterchangeably. OC-3c is sufficient to transmit a 6 MHz analog videochannel with a reasonable signal-to-noise ratio. The digital headendtransmitter also provides error-encoding to each digital video channeland frames the error-encoded digital video channels in groups of threeinto a SONET OC-3 bitstream. High quality digital video can betransmitted at an OC-1 bit-rate. N digital video channels can be framedinto an OC-N bit-rate. SONET bitstreams from M analog video channels andgroups of digital video channels are time-division multiplexed and sentat M times the OC-3 bit-rate through fiber optic cables from the headendto the nodes. In a MFPC system, the data is first broadcast from theheadend to the mux fiber nodes which further broadcast the data to themini fiber nodes. The mux-fiber nodes do not change the format of thedata.

The SONET bitstreams are demultiplexed at the nodes back to the OC-3bit-rate and deframed to recover the digital data. Digital datacorresponding to analog video channels is converted back to an analogformat. Digital data corresponding to digital video channels isdigitally modulated and converted to an analog format. Channels in theiranalog format are frequency-division multiplexed by modulation ontodesignated radio frequency carriers and distributed through coaxialcables to homes.

Information for interactive services, such as telephony or the Internet,originates from many locations and is not consistently transmitted overtime. Telephone calls are typically short in duration, averaging about 3minutes. Internet traffic duration averages over 30 minutes. Therefore,the ability to add or drop channels easily is advantageous. The digitalCATV network time-division multiplexes channels for transmission fromthe headend to the nodes. Time Division Multiplexing (TDM) allows formultiple locations from the headend to the nodes where channels can beeasily added or dropped as the need arises. Telephony and Internetservices are already built on the characteristics and performance of TDMtechnology.

Interactive services make the CATV network increasingly more symmetric,with as much information traveling upstream as downstream. Downstreamrefers to data that flows from the CATV network to the homes, andupstream refers to data that flows from the homes to the CATV network.In one embodiment, bandwidth for upstream data is allocated between 5MHz and 45 MHz as well as between 900 MHz and 1 GHz. Each headend serves10,000 to 300,000 or more homes. Each node serves a subset of the homesserved by the headend. It is advantageous to be able to add or drop dataat each node so that fewer homes share the allocated bandwidth forupstream data.

A location where data can be added or dropped is referred to as a “Pointof Presence” (POP). A POP links external data networks, including theInternet, cellular network, Public Switched Telephone Network (PSTN),and satellite network, to the digital CATV network. Information from theexternal data networks passes to the digital CATV network at the POP.Additionally, information from the digital CATV network can pass to theexternal data networks at the POP. For example, the headend or the nodecan serve as a POP. A bank of modems can be incorporated in each POP tointerface between the external data networks and the homes. The bank ofmodems can also pass information between the homes serviced by thedigital CATV network. Other locations in the digital CATV network, suchas the mux fiber node in the MFPC system, can also serve as a POP.Multiple POPs between the headend and the nodes provide the flexibilityto add or drop data that is common to multiple nodes.

A closed-loop digital CATV network increases the reliability of thenetwork due to feedback. Digital format includes extra bits, such asparity bits, to detect defects, errors, or failures in transmission.Remote indications control action in network protocols and bad packetscan be resent without interruption.

A digital CATV network is cost-efficient. Costly processing, such asForward Error Correction (FEC) of digital video channels, is performedat a headend. Standardized, thus economical, digital network equipmentis used throughout the network by framing digital data into standardizedbit-rates, such as OC-3, OC-12, OC-48, or OC-192. The ability to add ordrop channels at nodes increases the effective upstream bandwidthwithout installing more fiber optic cables from the headend to thenodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an illustration of a CATV network.

FIG. 1B is a detailed diagram of the CATV network illustrated in FIG.1A.

FIG. 2 is a graph showing spectral locations that different servicesoccupy in a broadband signal delivered by a CATV network to subscribers'homes.

FIG. 3 (shown as 3A and 3B) is a block diagram of an analog headendtransmitter, including frequency domain representation of waveforms atvarious points.

FIG. 4 is a block diagram of a digital headend transmitter.

FIG. 5A is a block diagram of one embodiment of a digitizer in thedigital headend transmitter shown in FIG. 4, including frequency domainrepresentation of waveforms at various points.

FIG. 5B is a block diagram of an alternate embodiment of a digitizer inthe digital headend transmitter shown in FIG. 4, including frequencydomain representation of waveforms at various points.

FIG. 6 is a block diagram of a formatter in the digital headendtransmitter shown in FIG. 4.

FIG. 7 is a block diagram of a digital node transmitter.

FIG. 8 is a block diagram of a deformatter in the digital nodetransmitter shown in FIG. 7.

FIG. 9 (shown as 9A and 9B) is a block diagram of a converter in thedigital node transmitter shown in FIG. 7, including frequency domainrepresentation of waveforms at various points.

FIG. 10 illustrates a method to distribute Internet protocal data in thedigital CATV network.

FIG. 11 illustrates a method to add or drop information in a digitalformat.

In the figures, the first digit of any three-digit number generallyindicates the number of the figure in which the element first appears.Where four-digit reference numbers are used, the first two digitsgenerally indicate the figure number.

DETAILED DESCRIPTION

The present invention involves the conversion of analog video channels,digital video channels and digital data into a digital format fortransmission via fiber optic cables to nodes in a digital CATV network.The analog video channels, digital video channels and digital data inthe digital format are converted to an analog broadband signal at thenodes for broadcast via coaxial cables to homes.

A digital CATV network system is illustrated in FIG. 1A. Informationfrom various sources, such as signals received by a satellite dish 103from a satellite 102 and signals from a video feed 104, are received ata headend 106. The headend 106 prepares the received information fortransmission to homes 131 (shown as 131A, 131J and 131N) through a cabledistribution system 125. Fiber optic cables 128 are typically used intransmission paths between the headend 106 and the cable distributionsystem 125. Coaxial cables 132 (shown as 132A, 132J and 132N) aretypically used in transmission paths between the cable distributionsystem 125 and respective homes 131. POPs 118, 120, 122 connect externaldata networks 114 to the headend 106 and various locations in the cabledistribution system 125. The external data networks 114 can include, forexample, the Internet, a PSTN, a cellular network and a satellitenetwork. The digital CATV network system is capable of providingservices simultaneously to, for example, a television 136, a telephone140, and a computer 144 inside the home 131A.

FIG. 1B is a detailed block diagram of the CATV network systemillustrated in FIG. 1A. In the headend 106, signals from the satellite102 and the video feed 104 are received by receivers 108, 109. Analogsignals from each receiver 108, 109 are provided to an analogmultiplexer 110. Digital signals from each receiver 108, 109 areprovided to a digital multiplexer 111. The multiplexed analog anddigital signals are provided to a digital headend transceiver 112. Thedigital headend transceiver 112 includes a digital headend transmitter113 and a digital headend receiver 115. Information at the headend 106is transmitted from the digital headend transmitter 113 to the cabledistribution system 125 via N fiber optic cables shown as fiber opticcables 128A-128N (collectively the fiber optic cables 128). Informationis received from the cable distribution system 125 by the digitalheadend receiver 115 via the fiber optic cables 128.

In one embodiment, the cable distribution system 125 includes N hubsshown as hubs 124A-124N (collectively the hubs 124). The hubs 124communicate with the headend 106 via the fiber optic cables 128. Eachhub 124 communicates with N nodes shown as nodes 126A-126N (collectivelythe nodes 126) via N fiber optic cables shown as fiber optic cables130A-130N (collectively the fiber optic cables 130). The nodes 126communicate with N homes shown as homes 131A-131N (collectively thehomes 131) via N coaxial cables shown as coaxial cables 132A-132N(collectively the coaxial cables 132). Each node 126 is connected tomultiple homes 131. Each home 131 is connected to one node 126. Forexample, the home 131A is connected to the node 126A via the coaxialcable 132A, the home 131J is connected to the node 126B via the coaxialcable 132J, and the home 131N is connected to the node 126N via thecoaxial cable 132N.

A digital node transceiver 146 process signals in each node 126. Thedigital node transceiver 146 includes a digital node transmitter 127 anda digital node receiver 129. The digital node transmitter 127 transmitsinformation to the homes 131 while the digital node receiver 129receives information from the homes 131. Information from the externaldata networks 114 can also be added at the various POPs 118, 120, 122for transmission in the cable distribution system 125.

In another embodiment, one or more of the hubs 124 are not directlyconnected to the headend 106 via the fiber optic cables 128. Instead,one or more of the hubs 124 are daisy-chained to another hub 124 whichhas a direct connection to the headend 106. Alternatively, the hubs 124can be connected in a ring configuration with a subset of the hubs 124directly connected to the headend 106. Similarly, the nodes 126 can beconnected in a ring configuration or daisy-chained with a subset of thenodes 126 directly connected to the hubs 124.

In an alternate embodiment, the cable distribution system 125 does notinclude the hubs 124. The headend 106 communicates with the nodes 126via the fiber optic cables 130. Each node 126 in the embodiment withoutthe hubs 124 typically services more homes 131 than each node 126 in theembodiment with the hubs 124. For convenience, subsequent discussions inthis application assume the cable distribution system 125 includes thehubs 124.

Inside the homes 131, various interfaces interpret the broadband signalfor processing by the intended equipment. For example, a set top box 134receives the video channels for display on the television set 136, anadapter 138 receives telephony data and adapts the signal from thecoaxial cable 132A to a twisted-pair telephone line 148, and a cablemodem 142 receives computer network data for the computer 144. A varietyof services, including interactive services, can share the same cablesand equipment in this digital CATV network.

Information received at the headend 106 for transmission in the cabledistribution system 125 can be in either analog or digital format. Forexample, analog video channels are typically received in 6 MHz wideanalog bands modulated onto an IF carrier, and digital video channelsare typically received as 8-bit MPEG data. The digital headendtransmitter 113 in the headend 106 converts the analog and digital videochannels to a digital format and combines the channels, using TDMtechnology, for transmission to the hubs 124 via the fiber optic cables128. The fiber optic cables 128 can be up to thousands of kilometers inlength. The hubs 124 further transmit the video channels encoded in thedigital format to the nodes 126 via the fiber optic cables 130.

The digital node transmitter 127 in each node 126 converts the videochannels encoded in the digital format back to their respective analogand digital format. The digital node transmitter 127 further modulatesthe video channels onto designated radio frequency carriers andfrequency-division multiplexes the channels into a broadband signal fortransmission to the homes 131 via the coaxial cables 132. In thisapplication, the locations where information goes through finalprocessing before being transmitted to the homes 131 are call the nodes126. In the CATV art, the connections between the nodes 126 and thehomes 131 are called the “last mile.” Typically, the last mile usescoaxial cables 132 and multiple homes 131 can be coupled to one coaxialcable 132. However, the present invention can be utilized in systemsthat use fiber optic cables, coaxial cables or a combination of both forall transmissions.

Since video channels are transmitted from the headend 106 to the nodes126 in the digital format using TDM technology, channels can be easilyadded or dropped between the headend 106 and the nodes 126. The headend106 serves many homes 131 (e.g., 50,000 to 300,000) and each hub 124serves a subset of those homes 131 (e.g., 5,000 or less to 50,000 ormore). The flexibility to add or drop channels at the hubs 124 allowtelevision programming to be customized for smaller regions.Furthermore, the ability to add or drop channels at the nodes 126 makesnarrowcasting possible. Narrowcasting customizes television programmingfor small groups. The nodes 126 serve fewer homes 131 than the headend106. Through narrowcasting, television programming can be tailored foreach neighborhood.

Information in a digital format from other sources can be easily addedto a video downstream. In one embodiment, information from the externaldata networks 114 can be added or dropped at the various POPs 118, 120,122 in the digital CATV network. The various POPs 118, 120, 122 includethe headend 106, the hubs 124, and the nodes 126. FIGS. 10 and 11,discussed later in this application, illustrate methods to combineinformation from various sources.

FIG. 2 is a graph showing spectral locations that different services canoccupy in a broadband signal delivered by the CATV network to the homes131. In one embodiment, upstream data occupies a first frequency band202 (e.g., between 5 MHz and 45 MHz). Downstream analog video channelsoccupy a second frequency band 204 (e.g., between 50 MHz and 550 MHz).Downstream digital video channels occupy a third frequency band 206(e.g., between 550 MHz and 750 MHz). Downstream Internet Protocol (IP)data occupies a fourth frequency band 208 (e.g., between 800 MHz and 900MHz). Upstream IP data occupies a fifth frequency band 210 (e.g.,between 900 MHz and 1 GHz).

Upstream information flows from the homes 131 to the cable distributionsystem 125. Downstream information flows from the cable distributionsystem 125 to the homes 131. Downstream analog video channels anddownstream digital video channels occupy the second frequency band 204and the third frequency band 206. In one embodiment, the secondfrequency band 204 and the third frequency band 206 take up 70% of a 1GHz broadband signal. Interactive services, including telephony andhigh-speed data access, occupy the first frequency band 202, the fourthfrequency band 208, and the fifth frequency band 210. Interactiveservices take up less than 30% of the 1 GHz broadband signal.

As demand grows for interactive services, a CATV network quickly runsout of bandwidth if too many homes 131 share the same frequency bands ina broadband signal. Therefore, it is advantageous to establish a digitalCATV network where the 1 GHz broadband signal is assembled at the nodes126 which serve a relatively small group of homes 131. The home 131Atied to the first node 126A does not have to share the availablebandwidth with the home 131J tied to the second node 126B. For example,interactive services are simultaneously delivered to the first home 131Atied to the first node 126A and the second home 131J tied to the secondnode 126B. By the nature of interactive services, the data packets goingto and from the first home 131A are distinct from the data packets goingto and from the second home 131J. The data packets flow through thecable distribution system 125 using TDM technology. The data packets aremodulated onto designated frequency carriers at the nodes 126 and becomepart of the broadband signal that is transmitted from the nodes 126 tothe homes 131. The data packet destined for the first home 131A occupiesa frequency carrier in a first broadband signal being broadcast from thefirst node 126A. The data packet destined for the second home 131J canoccupy the same frequency carrier in a second broadband signal beingbroadcast from the second node 126B. The first broadband signal is notreceived by the home 131J tied to the second node 126B, and the secondbroadband signal is not received by the home 131A tied to the first node126A. To conserve bandwidth, the data packet destined for the secondhome 131J does not unnecessarily occupy any bandwidth in the firstbroadband signal, and the data packet destined for the first home 131Adoes not unnecessarily occupy any bandwidth in the second broadbandsignal.

FIG. 3 (shown as 3A and 3B) is a block diagram of one embodiment of ananalog headend transmitter 320. Analog video channels A_(i)(t) andpartially-processed digital video channels D_(i)(t) are processed by Nrespective upconverters shown as upconverters 302A-302N (collectivelythe upconverters 302), followed by N respective Band Pass Filters (BPFs)shown as BFPs 304A-304N (collectively the BFPs 304). Each digital videochannel D_(i)[nT] is processed by a FEC encoder 312, a digital modulator316, and a Digital-to-Analog-Converter (DAC) 318 prior to processing bythe respective upconverter 302 and the respective BPF 304. Multipleanalog video channels and digital video channels are frequency-divisionmultiplexed in a combiner 306 after the above signal processing. Thefrequency-division multiplexed electrical signal S(ω) passes through anelectrical-to-optical converter 308 for transmission via the fiber opticcables 128.

In the analog headend transmitter 320, analog video channels A_(i)(t)are received at the headend 106 as IF signals. The analog video channelsare modulated onto respective designated radio frequency carriers afterpassing through the respective upconverters 302 and the respective BPFs304. Digital video channels D_(i)[nT] are received at the headend 106 indigital bits. Each digital video channel goes through extra processing.The digital bits pass through the FEC encoder 312 to reduce theprobability of errors in the transmission. The digital bits are thenprovided to the digital modulator 316 and the DAG 318. The outputD_(i)(t) of the DAC 318 is an analog equivalent of the digital videochannel. The DAC outputs are modulated onto respective designated radiofrequency carriers after passing through the respective upconverters 302and the respective BPFs 304. The analog and digital video channels arefrequency-division multiplexed in the combiner 306 into a broadbandsignal S(ω). The broadband signal S(ω) is converted to an optical signalby the electrical-to-optical converter 308. The optical signal istransmitted from the headend 106 via the fiber optic cables 128.Frequency Division Multiplexing (FDM) does not always facilitate easyadding or dropping of channels. The adding or dropping process can becostly. Thus, in one embodiment, channels are only added or dropped atthe headend 106.

The present invention uses the digital headend transmitter 113 shown inFIG. 1B. A block diagram of one embodiment of the digital headendtransmitter 113 is shown in FIG. 4. Analog video channels A_(i)(t) areprovided to a digitizer 402. The digital bits Z_(i)[0:b1] at the outputof the digitizer 402 are provided to a framer 404. N groups of digitalvideo channels D_(i)[nT] are provided to N respective formatters shownas formatters 408A-408N (collectively the formatters 408). The digitaloutputs I_(i)[0:b3] of the framer 404 and the digital outputsG_(i)[0:b3] of the formatters 408 are combined in a multiplexer 406. Thecombined digital signal C[0:b3] is converted from a parallelrepresentation to a serial representation in a serializer 410. Theserial digital signal at the output of the serializer 410 passes throughan electrical-to-optical converter 412 in preparation for transmissionvia the fiber optic cables 128.

Analog video channels A_(i)(t) are individually sampled and decimated inthe digitizer 402. The digital bits Z_(i)[0:b1] of each analog videochannel are arranged in a digital format in the framer 404. In oneembodiment, SONET is used as the digital format. SONET is a standard foroptical telecommunications transport. The standard allows equipment fromdifferent suppliers to be used in a fiber system. However, otherstandards (e.g., Asynchronous Transfer Mode or Fiber Channel (FC)) canbe used in conjunction with SONET. SONET format advantageously requiresa relatively small amount of additional bits to be added to raw data.SONET overhead is approximately 3% of the raw data. In anotherembodiment, SONET data can be segmented and incorporated into systemusing FC protocol by adding FC frames around SONET data.

In one embodiment, the bandwidth of each analog video channel A_(i)(t)is 6 MHz wide, and the bit-rate of the corresponding digitalrepresentation Z_(i)[0:b1] at the output of the digitizer 402 is 129.6Mega-Bits-Per-Second (Mbps). Each digitized analog video channel cannotfit directly into a single 52 Mbps OC-1 payload. Therefore, the digitalbits Z_(i)[0:b1] are framed into an OC-3c payload which is a 155.52 Mbpsbitstream. The “c” appended to “OC-3” signifies that envelope capacitiesfrom three OC-1s have been concatenated to transport one signal.

In one embodiment, the digital video channels D_(i)[nT] are introducedto the digital headend transmitter 113 in the form of digital bits, suchas 8-bit MPEG data. Groups of digital video channels are provided to therespective formatters 408. The formatters 408 provide error-encoding(e.g., FEC) for each individual digital video channel. Then theformatters 408 frame respective groups of digital video channels into adigital format. Each digital video channel can be framed individuallyinto an OC-1 bit-rate or N digital video channels can be framed togetherinto an OC-N bit-rate. The formatters 408 can process digital data fromthe external data networks 114 in a similarly manner as digital videochannels. Digital data is also error-encoded and framed before it isadded to other data in the downstream flow.

The digital CATV network can be more economical using a higher bit-rate.At the same time, it is advantageous to frame at a low bit-rate toprovide relatively more flexibility in the dropping and adding ofchannels. For example, increased flexibility to drop or add channelsfacilitates narrowcasting. In one embodiment, three digital videochannels are framed together into an OC-3 bit-rate. Network equipmentfor processing OC-3 bit-rate signals is widely available and inexpensivedue to economy of scale. Standard network equipment is also available toprocess bit-rates of OC-12, OC-48, and OC-192.

Information is represented by eight-bit wide digital bytes in a SONETformat. In one embodiment, the digital outputs I_(i)[0:7] of the framer404 and the digital outputs G_(i)[0:7] of the formatters 408 are in theSONET format using an OC-3 bit-rate. The digital outputs I_(i)[0:7] andG_(i)[0:7] are combined using TDM technology in the multiplexer 406. Thecombined digital signal C[0:7] is converted from an 8-bit parallelrepresentation to a serial representation in the serializer 410.

FIG. 5A is a block diagram of one embodiment of the digitizer 402 usedto digitize and decimate each analog video channel A_(i)(t) in thedigital headend transmitter 113. Each analog video channel A_(i)(t) isprovided to a downconverter 502 followed by a BPF 504. The outputW_(i)(t) of the BPF 504 is provided to an Analog-to-Digital Converter(ADC) 506. The digital output X_(i)[nT] of the ADC 506 is provided to adigital mixer 508. The output Y_(i)[nT] of the digital mixer 508 isprovided to an anti-aliasing digital filter 510 followed by a samplerate compressor 512. The output Z_(i)[nT] of the sample rate compressor512 is a digitized version of the analog video channel A_(i)(t).

In one embodiment, an analog video channel A_(i)(t) is a 6 MHz wide IFsignal. A spectral plot A_(i)(ω) 581 shows the analog video channeloccupying a bandwidth between 40 MHz and 46 MHz. A_(i)(t) is frequencyshifted to a second IF by the downconverter 502. The output V_(i)(t) ofthe downconverter 502 is provided to the BPF 504 to remove unwantedspectral images. It is more advantageous to downconvert A_(i)(t) to thesecond IF rather than to a baseband frequency. Unwanted spectral imagesare spectrally further from the desired signal in downconversion to thesecond IF. Thus, performance requirements for the subsequent BPF 504 areless stringent.

In one embodiment, a ten-bit ADC 506 is used to digitize the second IFsignal W_(i)(t) at the output of the BPF 504. Ten bits typically providean acceptable signal-to-noise ratio (SNR) in the cable distributionsystem 125. Fewer bits cause noticeable degradation to the overallperformance of the digital CATV network. More bits decrease thethroughput with no significant improvement in performance.

For Nyquist sampling, the sampling frequency Fs is at least twice thehighest frequency of a signal. In one embodiment, the second IF signalW_(i)(t) at the output of the BPF 504 is between 6 MHz and 12 MHz. Thesampling frequency Fs of the ADC 506 is 25.92 MHz, which is greater thantwice the highest frequency of the second IF signal W_(i)(t). Thedigital output X_(i)[nT] of the ADC 506 is provided to the digital mixer508 to frequency shift the sampled signal to a baseband frequency.

Spectral images of a signal repeat at f±nFs after sampling, where f isthe frequency of the signal being sampled, and n is a positive integer.The digital baseband signal Y_(i)[nT] at the output of the digital mixer508 is provided to the anti-aliasing digital filter 510 followed by thesample rate compressor 512. The anti-aliasing digital filter 510 isconfigured to suppress frequencies that can otherwise overlap afterprocessing by the sample rate compressor 512. The sample rate compressor512 causes the repeating spectral images of the sampled signal to bespectrally closer together. The degree of closeness is determined by adecimation factor. The decimation factor is a positive integer. Thesample rate compressor 512 increases data throughput, thus allowing morechannels to be simultaneously broadcast, in the cable distributionsystem 125 by transmitting a subset of the sampled signal. The signalZ_(i)[nT] at the output of the sample rate compressor 512 has aneffective sampling frequency that is lower than the sampling frequencyFs of the ADC by a factor equivalent to the decimation factor. Signalsare typically over-sampled. Signal integrity is maintained in the cabledistribution system 125 so long as the effective sampling frequency atthe output of the sample rate compressor 512 satisfies the Nyquistcriterion.

In one embodiment, a 6 MHz analog video channel is sampled by a ten-bitDAC 506 using a sampling frequency Fs of 25.92 MHz. A decimation factorof two is used by the sample rate compressor 512 to reduce the number ofsamples by half Every other sample is provided to the cable distributionsystem 125. The effective sampling frequency of the transmitted signalis 12.96 MHz, half of Fs. The effective sampling frequency, 12.96 MHz,is more than twice the analog video channel bandwidth, 6 MHz. Thus, theNyquist rate is satisfied, and the signal can be accurately transmittedusing half of the samples. Using the sampling frequency of 25.92 MHz,the ten-bit DAC 506, and the decimation factor of two, the bitthroughput for each analog video channel is 129.6 Mbps from the outputof the digitizer 402 (25.92 MHz x 10 bits/2).

FIG. 5B is a block diagram of an alternate embodiment of the digitizer402 used to digitize and decimate each analog video channel A_(i)(t) inthe digital headend transmitter 113. Each analog video channel A_(i)(t)is provided directly to an ADC 514. The output of the ADC 514 isprovided to a half-complex mixer 518. The half-complex mixer 518produces two outputs which are provided to respective anti-aliasingdigital filters 520A, 520B followed by respective sample ratecompressors 522A, 522B. The outputs from the respective sample ratecompressors 522A, 522B are provided to an interleaver 524.

In one embodiment, the analog video channel A_(i)(t) is an IF signallying in a 6 MHz band of 40 to 46 MHz as illustrated by a spectral plot591. A ten-bit ADC 514 undersamples the IF signal such that no aliasingoccurs. A spectral plot 592 illustrates undersampling at 59.2 MHz. Thehalf-complex mixer 518 frequency shifts the IF signal to a basebandfrequency and outputs a complex signal with an in-phase (I) componentand a quadrature-phase (Q) component. A spectral plot 594 illustratesthe complex baseband signal. The anti-aliasing digital filter 520A andthe sample rate compressor 522A filter and decimate the I component. TheQ component is similarly filtered and decimated by the anti-aliasingdigital filter 520B and the sample rate compressor 522B. Finally, theinterleaver 524 interleaves the decimated I and Q components inpreparation for framing into a digital format.

FIG. 6 is a block diagram of one of the formatters 408 used toerror-encode and frame a group of digital video channels in oneembodiment of the digital headend transmitter 113. The group of Ndigital video channels are provided to N respective FEC encoders shownas FEC encoders 606A-606N (collectively the FEC encoders 606). In oneembodiment, each FEC encoder 606 includes a Reed-Solomon encoder 602, aninterleaver 604, a randomizer 608, and a trellis encoder 612. The groupof individually error-encoded digital video channels is combined in aframer 610. Digital data from the external data networks 114 can beprocessed by the formatters 408 in a similar manner. In addition to theheadend 106, the formatters 408 can reside in other POPs 120, 122 toreceive and prepare digital data for addition to the downstreamtransport.

In one embodiment, the digital video channels D_(i)[nT] are presented inthe form of eight-bit MPEG datastreams. The digital bits are provided tothe FEC encoders 606. A simple FEC scheme is to send redundant signalbits. This simple FEC scheme is effective but relatively less efficient.More complex coding has been developed to provide FEC with a minimal setof extra bits. In one embodiment, the Reed-Solomon encoder 602 is usedfor the FEC. The Reed-Solomon encoder 602 provides block encoding andcorrects multiple symbols within a block. The interleaver 604 evenlydisperses the symbols and enables the correction of burst noise inducederrors. The randomizer 608 provides for even distribution of the symbolsin a constellation. The trellis encoder 612 allows the introduction ofredundancy to improve the threshold SNR by increasing the symbolconstellation without increasing the symbol rate. Individually encodeddigital video channels are combined with other similarly encoded digitalvideo channels in the framer 610. The combined signal is in a digitalformat. In one embodiment, the digital format is a SONET format with anOC-N bit-rate, where N denotes the number of individual digital videochannels in the combined signal.

The FEC encoders 606 largely address transmission errors from the nodes126 to the homes 131, where analog and digital video channels arefrequency-division multiplexed into a broadband signal and sent throughthe coaxial cables 132. The set top box 134 and the cable modem 142inside the homes 131 perform the FEC decoding. Digital transmission fromthe headend 106 to the nodes 126 benefits from a network protocol thatautomatically monitors errors. Therefore, the error-encoding process ofthe digital video channels can take place at the nodes 126 withoutjeopardizing its functionality. However, encoding at multiple nodes 126instead of at the single headend 106 incurs relatively more cost withoutsignificant benefit.

FIG. 7 is a block diagram of one embodiment of the digital nodetransmitter 127. Optical digital data is received from the fiber opticcables 130 and transformed to electrical digital data by anoptical-to-electrical converter 702. The electrical digital data istypically provided to a serial-to-parallel converter 704 to allowprocessing as bytes. The output of the serial-to-parallel converter 704is provided to a demultiplexer 706 to separate digital data that hasbeen previously combined by the multiplexer 406. The digital outputs ofthe demultiplexer 706 corresponding to analog video channels areprovided to a deframer 708. The digital outputs of the demultiplexer 706corresponding to N groups of digital video channels are provided to Nrespective deformatters shown as deformatters 712A-712N (collectivelythe deformatters 712). The digital outputs or the deframer 708 and thedeformatters 712 are provided to a converter 710 to represent thedigital signals in an analog format. The output of the converter 710 isan analog broadband signal S(t) suitable for transmission to the homes131 via the coaxial cables 132.

The digital node transmitter 127 accepts TDM data in a digital formatand converts the TDM data into FDM data in an analog format. In oneembodiment, the digital node transmitter 127 resides in the node 126.This provides the optimal signal quality in the digital CATV network.Information is transmitted through most of the digital CATV network,from the headend 106 to the node 126, in a digital format.Error-monitoring is inherent in the digital CATV network. Signal qualityis high as there is no degradation in error-free digital data. At thesame time, placing the digital node transmitter 127 in the node 126optimizes the reliability of the digital CATV network by minimizing thedistance information is transmitted in analog format. Finally, placingthe digital node transmitter 127 as close to the homes 131 aseconomically feasible maximizes the number of POPs 118, 120, 122 in thedigital CATV network where services can be added or dropped with ease.

The digital node transmitter 127 receives optical digital data that istransmitted serially through the fiber optic cable 130. Theoptical-to-electrical converter 702 transforms the optical digital datato electrical digital data for processing. The serial digital data atthe output of the optical-to-electrical converter 702 is accumulatedinto parallel bits. In one embodiment, the serial bits are assembledinto eight parallel bits to recover the eight-bit bytes that wereconverted to serial bits by the serializer 410 of the digital headendtransmitter 113. The eight-bit bytes are then provided to thedemultiplexer 706 to recover the individual analog video channels andthe grouped digital video channels that were combined using TDMtechnology by the multiplexer 406 of the digital headend transmitter113.

The individual analog video channels are provided to the deframer 708.The deframer 708 removes the extra bits appended to the raw data forerror-monitoring and status indication in the digital CATV network. Thedigital outputs Z_(i)[nT] of the deframer 708 are the same as thedigital samples produced by the digitizer 402 of the digital headendtransmitter 113. In one embodiment, the digitizer 402 produces aninterleaved I and Q output and the deframer 708 de-interleaves the I andQ components for subsequent processing by the converter 710. Theconverter 710 unsamples the digital outputs of the deframer 708 torecover the analog format of each analog video channel.

The groups of digital video channels are provided to the respectivedeformatters 712 to separate into individual digital video channels, andto prepare the digital video channels for conversion to an analogformat. The converter 710 converts the digital video channels from thedigital format to the analog format. The converter 710 also combines theanalog video channels and the digital video channels in their analogformat into one analog broadband signal S(t) using FDM. The format ofS(t) is identical to signals that are presently transmitted to the homes131. Therefore, the digital CATV network can be seamlessly implemented.The existing set top box 134, adapter 138, and cable modem 142 in thehomes 131 can still be used.

FIG. 8 is a block diagram of one embodiment of the deformatters 712 inthe digital node transmitter 127. Data G_(i)[nT] in a digital format,representing a group of digital video channels, is provided to adeframer 802. N outputs of the deframer 802, representing N digitalvideo channels, are provided to N respective modulators shown asmodulators 806A-806N (collectively the modulators 806). The deframer 802ungroups the digital video channels in addition to removing extra bitsutilized for transport in the digital CATV network. Digital modulationis introduced by the modulators 806 to prepare the digital data fortransmission in the analog format. Digital modulation schemes, includingamplitude shift keying, phase shift keying and frequency shift keying,can be used. In one embodiment, quadrature amplitude modulation isemployed.

FIG. 9 (shown as 9A and 9B) is a block diagram of one embodiment of theconverter 710 in the digital node transmitter 127 of FIG. 7. Thedigitized data Z_(i)[nT]of analog video channels is provided to theconverter 710. The digitally modulated data Q_(i)[nT] of digital videochannels can be similarly provided to the converter 710. The digitaldata of N analog or digital video channels are provided to N respectivesample rate expanders shown as sample rate expanders 902A-902N(collectively the sample rate expanders 902) followed by N respectiveanti-imaging filters shown as anti-imaging filters 904A-904N(collectively the anti-imaging filters 904). Outputs F_(i)[nT] from Ngroups of the anti-imaging filters 904 are combined by N respectivedigital frequency modulator blocks shown as digital frequency modulatorblocks 906A-906N (collectively the digital frequency modulator blocks906). The combined digital signals J_(i)[nT] are provided to Nrespective DACs shown as DACs 908A-908N (collectively the DACs 908). Theanalog signals K_(i)(t) at the output of the DACs 908 are provided to Nrespective LPFs shown as LPFs 910A-910N (collectively the LPFs 910). Theoutputs P_(i)(t) of the LPFs 910 are provided to N respectiveupconverters shown as upconverters 912A-912N (collectively theupconverters 912). The outputs Q_(i)(t) of the upconverters 912 areprovided to N respective BPFs shown as BFPs 914A-914N (collectively theBFPs 914). A combiner 916 uses FDM technology to combine outputs inanalog format from the BPFs 914 into one analog broadband signal S(t).

In one embodiment, the digital frequency modulator blocks 906 areInverse Fast Fourier Transform (IFFT) blocks. The IFFT blocks 906provide a more cost-efficient converter 710. Each IFFT block 906combines a group of analog or digital video channels in the digitaldomain using FDM technology. Fewer DACs 908, LPFs 910, upconverters 912and BPFs 914 are required. The sample rate expanders 902 and theanti-imaging filters 904 prepare the analog or digital video channelsfor combination without overlap. In one embodiment, the digital data ofeach analog or digital video channel is interpolated by an integerfactor of K and passed through the anti-imaging filters 904. A spectralplot Z_(i)[ω] 981 shows that spectral images of the digital data repeatat multiples of the sampling frequency Fs. A spectral plot F_(i)[ω] 982of the output of the anti-imaging filters 904 shows that interpolationby K and anti-image filtering effectively change the repetition rate tomultiples of K times Fs. A spectral plot J_(i)[ω] 983 of the output ofthe IFFT blocks 906 illustrates the FDM of K analog or digital videochannels. The upper limit on the number of channels that can befrequency-division multiplexed by the IFFT blocks 906 depends on thespeed of the DACs 908. The higher speed DACs 908 allow the IFFT blocks906 to frequency-division multiplex more analog or digital videochannels. The LPFs 910 after the DACs 908 remove unwanted spectralimages in the analog outputs of the DACs 908. The upconverters 912 andthe BPFs 914 frequency shift the analog signal to a designated frequencycarrier. Each group of channels is frequency shifted to a differentfrequency carrier. Multiple groups of channels are combined into one FDMsignal S(t) for broadcast to the homes 131.

FIG. 10 illustrates a method to distribute computer network data (e.g.,IP data) in the digital CATV network. Various sources (e.g., the headend106, the nodes 126 or the external data networks 114) communicate the IPdata to one of the nodes 126 or other POP 118, 120, 122. The POP 118,120, 122 includes a process or 1010, one or more transceivers 1012,1014, 1016, a bank of N modems shown as modems 1002A-1002N (collectivelythe modems 1002), N couplers shown as couplers 1004A-1004N (collectivelythe couplers 1004). Various sources communicate with the transceivers1012, 1014, 1016. For example, the headend 106 communicates with thetransceiver 1014, the nodes 126 communicate with the transceiver 1016,and the external data networks 114 communicate with the transceiver1012. The transceivers 1012, 1014, 1016 communicate with the processor1010. The processor 1010 communicates with the modems 1002. The modems1002 communicate with respective couplers 1004. Video downstream data1008 is broadcast to the couplers 1004. The couplers 1004 communicatewith N respective locations shown as locations 1006A-1006N (collectivelythe locations 1006).

Downstream IP data is received by the transceivers 1012, 1014, 1016 fromthe various sources. The transceivers 1012, 1014, 1016 forward thedownstream IP data to the processor 1010. Each packet of the IP datatypically includes an address indicating its intended destination. Theprocessor 1010 processes the downstream IP data and routes thedownstream IP data to the appropriate modems 1002 according to theaddresses of the respective packets. The modems 1002 forward thedownstream IP data packets to the respective couplers 1004 whichcommunicate the information to respective locations 1006. Each of thelocations 1006 represents a group of homes 131 serviced by the digitalCATV network.

Upstream IP data from the homes 131 can be provided to the digital CATVnetwork for distribution. In addition to combining the video downstreamdata 1008 with the downstream IP data from the modems 1002 fortransmission to the respective locations 1006, the couplers 1004 receivedata from the respective locations 1006 and provide the upstream IP datato the modems 1002. The modems 1002 forward the upstream IP data to theprocessor 1010. The processor 1010 processes the upstream IP data androutes the IP data packets according to respective destinationaddresses. For example, the processor 1010 routes the IP data packetback to one of the modems 1002 as downstream IP data when the addressindicates that the destination is one of the homes 131 serviced by thatparticular POP 118, 120, 122. Alternatively, the processor 1010 routesthe IP data packet to the transceiver 1012 when the address indicatesthat the destination is one of the external data networks 114. Theprocessor 1010 routes the IP data packet to the transceiver 1016 whenthe address indicates that the destination is one of the homes 131serviced by another node 126 that is coupled to the POP 118, 120, 122.Finally, the processor 1010 routes the IP data packet to the transceiver1014 when the address indicates one of the other destinations. Thetransceivers 1012, 1014, 1016 can be a combination of opticaltransceivers, electrical transceivers or wireless transceivers dependingon whether fiber optic cables, coaxial cables or wireless links are usedto couple the various sources to the transceivers 1012, 1014, 1016.

The modems 1002 facilitate the distribution of the IP data from varioussources to the homes 131 and the transmission of the IP data between thehomes 131 serviced by the digital CATV network. Both the upstream IPdata and the downstream IP data are processed and routed by theprocessor 1010. By utilizing the bank of modems 1002 and correspondingcouplers 1004, the IP data packets destined for the different locations1006 can occupy the same time slot or frequency band. The effectivebandwidth for the group of locations 1006 is increased.

FIG. 11 illustrates one embodiment of a method to add or dropinformation in a digital format. A network element 1102 receives a firstbitstream 1110 for processing. The network element 1102 has N portsshown as ports 1104A-1104N (collectively the ports 1104). Informationdesignated to be dropped from the first bitstream 1110 can be madeavailable at one or more of the ports 1104. Information to be added tothe first bitstream 1110 is made available to one of the ports 1104. Thenetwork element 1102 accesses the information that is to be dropped orinserted in the first bitstream 1110. Information from the firstbitstream 1110 that is not dropped continues through the network elementwithout requiring special pass-through units or other signal processing.The network element 1102 outputs a second bitstream 1112 that containsthe information of the first bitstream 1110 without the droppedinformation but includes the inserted information. In one embodiment,the network element 1102 is a SONET Add/Drop Multiplexer (ADM). The ADMcan consolidate information from many locations.

Although described above in connection with particular embodiments ofthe present invention, it should be understood the descriptions of theembodiments are illustrative of the invention and are not intended to belimiting. Various modifications and applications may occur to thoseskilled in the art without departing from the true spirit and scope ofthe invention as defined in the appended claims.

1. A digital cable television network comprising: a headend transmittercomprising: a digitzer configured to receive an analog video signal andconvert said analog video signal to corresponding digital values; and aframer configured to receive said digital values of said analog videosignal and produce an output in a digital format, said digital formatconfigured to include digital bits to facilitate a monitoring of errorsand an indication of status in a transmission of said analog videosignal in said digital format; and a node transmitter located at a nodeclosest to a subscriber and comprising: a deframer configured to receivesaid digital format of said analog video signal and recover said digitalvalues; and a converter configured to receive outputs from said deframerand produce an analog signal suitable for a subscriber type receiver. 2.The digital cable television network of claim 1, wherein said digitalformat is a Synchronous Optical Network standard and said analog videosignal is represented by a level 3 bit-rate.
 3. A digital cabletelevision network comprising: a formatter located at a headend andconfigured to receive a digital video signal, provide forwarderror-correction for said digital video signal, and produce an output ina digital format, wherein said digital format is configured to includedigital bits to facilitate a monitoring of errors and an indication ofstatus in a transmission of said digital video signal in said digitalformat; and a node transmitter located at a node closest to subscribersand comprising: a deformatter configured to receive said digital formatof said digital video signal, recover said digital video signal withsaid forward error-correction, and provide digital modulation; and aconverter configured to receive outputs from said deformatter andproduce an analog signal suitable for a subscriber type receiver.
 4. Thedigital cable television network of claim 3 wherein: said digital formatis a Synchronous Optical Network standard; said digital video signal isrepresented by a level 1 bit-rate in said Synchronous Optical Networkstandard; and said formatter combines a plurality of said digital videosignals to achieve a standardized bit-rate.
 5. The digital cabletelevision network of claim 4 wherein said standardized bit-rate is alevel 3 bit-rate.
 6. A digital cable television network comprising: aheadend transmitter located at a headend and configured to receive asignal and produce a digital signal in a digital format, wherein saiddigital format is configured to include digital bits to facilitate amonitoring of errors and an indication of status in a transmission ofsaid signal in said digital format; and a node transmitter located at anode closest to subscribers and configured to receive said signal insaid digital format and produce an output in an analog format suitablefor a subscriber type receiver.
 7. The digital cable television networkof claim 6 wherein a plurality of said signals in said respectivedigital formats comprises a set of downstream data, said set ofdownstream data is transmitted to said node using time divisionmultiplexing technology, an external network signal in said digitalformat is added to said set of downstream data by a standard networkelement, and content of said set of downstream data is modified by saidstandard network element.
 8. The digital cable television network ofclaim 7, wherein said standard network element is a SONET Add/DropMultiplexer.
 9. The digital cable television network of claim 6 whereina bank of modems is configured to transmit information between anexternal network and said digital cable television network.
 10. Thedigital cable television network of claim 9 wherein said information isInternet protocol data.
 11. The digital cable television network ofclaim 6 wherein a bank of modems is configured to transmit informationbetween subscribers of said digital cable cable television network. 12.The digital cable television network of claim 6 wherein said headend isa point of presence.
 13. The digital cable television network of claim 6wherein said node is a point of presence.
 14. The digital cabletelevision network of claim 6, further comprising a hub and wherein saidhub is a point of presence.
 15. The digital cable television network ofclaim 6, wherein said digital format is a Synchronous Optical Networkstandard.
 16. The digital cable television network of claim 6, whereinsaid digital format is a Fiber Channel standard and said signal isrepresented in said Fiber Channel standard by segmented SONET frame. 17.A digital cable television network comprising: a headend transmittercomprising: an analog-to-digital converter configured to receive ananalog video signal and produce corresponding digital values at fixedtime intervals; a digital mixer configured to receive an output of saidanalog-to-digital converter and frequency shift to a desired basebandfrequency; an anti-aliasing digital filter configured to receive anoutput of said digital mixer, a sample rate compressor configured toreceive an output of said digital filter; a framer configured to receivea digital signal from said sample rate compressor, said framerconfigured to arrange digital signal to achieve a digital formatconfigured to include additional digital bits to facilitate a monitoringof errors and an indication of status in a transmission of said digitalsignal in said digital format; and a node transmitter located at a nodeclosest to subscribers and comprising: a deframer configured to recoversaid digital signal embedded in said digital format; a sample rateexpander configured to receive an output of said deframer; ananti-imaging filter configured to receive an output of said sample rateexpander; a digital frequency modulator block configured tofrequency-division multiplex two or more digital signals from outputs ofrespective anti-imaging filters; a digital-to-analog converterconfigured to receive an output of said digital frequency modulatorblock; a lowpass filter configured to receive an output of saiddigital-to-analog converter, said lowpass filter configured to pass abaseband frequency; an upconverter configured to receive an output ofsaid lowpass filter and frequency shift to a desired transmit frequency;and a transmit bandpass filter configured to receive an output of saidupconverter, said transmit bandpass filter configured to pass saiddesired transmit frequency.
 18. The digital cable television network ofclaim 17 wherein said digital frequency modulator block comprises anInverse Fast Fourier Transform block.
 19. A digital network systemcomprising: a headend transmitter comprising: a forward error-correctionencoder configured to receive a digital video signal; a framerconfigured to receive digital signals from two or more forwarderror-correction encoders, said framer configured to arrange saiddigital signals to achieve a digital format configured to includeadditional digital bits to facilitate a monitoring of errors and anindication of status in a transmission of said digital signal in saiddigital format; and a node transmitter located at a node closest tosubscribers and comprising: a deframer configured to recover saiddigital signals embedded in said digital format; a digital modulatorconfigured to receive an output of said deframer; a sample rate expanderconfigured to receive an output of said digital modulator; ananti-imaging filter configured to receive an output of said sample rateexpander; a digital frequency modulator block configured tofrequency-division multiplex two or more outputs of respectiveanti-imaging filters; a digital-to-analog converter configured toreceive an output of said digital frequency modulator block; a lowpassfilter configured to receive an output of said digital-to-analogconverter, said lowpass filter configured to pass a baseband frequency;an upconverter configured to receive an output of said lowpass filterand frequency shift to a desired transmit frequency; and a transmitbandpass filter configured to receive an output of said upconverter,said transmit bandpass filter configured to pass the desired transmitfrequency.
 20. The digital cable television network of claim 19 whereinsaid digital frequency modulator block comprises an Inverse Fast FourierTransform block.
 21. A digital cable television network comprising:means for converting a signal to a digital format at a headend, whereinsaid digital format is configured to include digital bits to facilitatea monitoring of errors and an indication of status in a transmission ofsaid signal in said digital format; and means for converting said signalin said digital format to an analog format at a node closest to asubscriber, wherein said analog format is suitable for a subscriber typereceiver.
 22. The digital cable television network of claim 21, furthercomprising: means for providing forward error-correction to said signalat said headend; and means for providing digital modulation to saidsignal with forward error-correction at said node.
 23. The digital cabletelevision network of claim 21, further comprising means for combining aplurality of said signals in said digital format to achieve astandardized bit-rate.
 24. The digital cable television network of claim21 wherein a plurality of said signals in said digital format comprisesa set of downstream data and said set of downstream data is transmittedto said node using time division multiplexing technology.
 25. Thedigital cable television network of claim 24, further comprising meansfor adding a signal from an external network to said set of downstreamdata.
 26. The digital cable television network of claim 21, furthercomprising means for transmitting information between subscribers ofsaid digital cable television network.
 27. A method of cable televisiontransmission comprising the acts of: converting a signal to a digitalformat at a headend, wherein said digital format is configured toinclude digital bits to facilitate error-monitoring and statusindication in a transmission of said video channel in said digitalformat; transmitting a first plurality of signals in said digital formatusing time division multiplexing technology; converting said signal insaid digital format to an analog format at a node closest to asubscriber, wherein said analog format is suitable for a subscriber typereceiver, and transmitting a second plurality of signals in said analogformat to a subscriber using frequency division multiplexing technology.28. The method of cable television transmission of claim 27, furthercomprising the acts of: providing forward error-correction to saidsignal at said headend; and providing digital modulation to said signalwith forward error-correction at said node.
 29. The method of cabletelevision transmission of claim 27, wherein said first plurality ofsignals in said digital format includes a signal from an externalnetwork and said second plurality of signals in said analog format is asubset of said first plurality of signals.